The present invention relates to an electrochromic display device.
In conventional electrochromic display elements a layer of electrochromic substance, either liquid or solid, is sandwiched between a pair of opposed glass members. On one of the glass plates is provided a counter electrode and on the other electrode is one or more individual electrodes having a particular display pattern. These electrodes are formed of a transparent conductive material such as In.sub.2 O.sub.3 and SnO.sub.2. Various shortcomings exist in conventional electrochromic display devices that affect their life time and quality.
When a potential is applied to a given display electrode, light from a chemical reaction occurs in the electrochromic material adjacent thereto for developing a given color or bleaching it, depending on the polarity of the applied voltage. Oxidation or reduction is correspondingly triggered in areas adjacent to the counter plate electrode. To assure stability in such reversible reaction the couuter plate electrode is formed of a metal oxide, such as iron oxides. However, after 10.sup.4 to 10.sup.6 times of device operation, it has been found that device tends to manifest a drift in field potential in areas adjacent to the counter plate electrode. This in turn prevents the necessary field potential from being developed in areas adjacent to pattern electrodes. The life time is also considered to be affected by a current which arises from a side effect of the reversible actions of the electrochromic substance. It is desirable that no current flow when the device attains a certain level of intensity.
Color shading is a predominant factor that determines the quality of a multi-segment electrochromic display device. If use is made of a solid electrochromic substance such as WO.sub.3, variations in layer thickness and evaporation condition from one layer to another tend to produce a wide range of different shades of color. Furthermore, the image retaining power of a given element tends to be affected by the formation of a color in an adjacent display element. To avoid this, a complicated circuit was needed.